Previous observations that muscle F-actin depolymerized rapidly when added to extracts of Acanthamoeba castellanii while Acanthamoeba F-actin was stable had led to the tentative identification of a depolymerizing factor and an ATP-sensitive stabilizing factor in the extract that interacted differently with muscle and Acanthamoeba F-actin. Although not yet fully explained, it now seems likely that most, if not all, of these observations can be explained by some previously known and some previously unknown properties of actophorin, an Acanthamoeba protein (discovered several years ago by another laboratory) that severs F-actin and sequesters actin monomers. We find that actophorin has much greater severing activity for muscle Ca-F-actin than for Acanthamoeba Ca-F-actin and others had previously shown that actophorin has a higher affinity for ADP-G-actin than for ATP-G-actin. Thus, actophorin may be the depolymerizing factor and G-actin the ATP-sensitive stabilizing factor in the amoeba extracts. A stable actophorin-ADP-G-actin complex in the extract would be dissociated by addition of ATP and the released actophorin would sever and depolymerize muscle Ca-F-actin more rapidly than Acanthamoeba Ca-F-actin.